This invention involves a microscope objective having a linear magnification of between 32.times. and 60.times., an aperture of at least 0.5, and a device that can set the objective to accommodate different cover-glass thicknesses. For any given aperture, a microscope objective design must take into account different thicknesses of cover-glasses to be used, since presence of a cover-glass, or changes in the thickness of any cover-glass that is present, influences the spherical aberration of the objective.
Since even small differences in cover-glass thicknesses noticeably impair the microscopic image for higher aperture objectives, correction mounts have been used to set these objectives to accommodate different cover-glass thicknesses. Such correction mounts, as described, for example, in Austrian Patent No. 170000 (corresponding Great Britain Patent No. 660,865) and West German Publication No. 2602730 (corresponding U.S. Pat. No. 4,059,342), change the air separation between two lenses of the objective. Generally, the rear lens is moved for this, while the front lens remains stationary. This varies the spherical aberration of the objective and thus compensates for the influence of different thicknesses of cover-glass.
Objectives are also known with correction mounts that permit a setting adjustment over a relatively large range of cover-glass thicknesses of from 1 to 2 mm. Such objectives are used, for example, in cellular research where objects are placed on inverse microscopes and observed through the bottom of a culture vessel. Objectives for this purpose are described, for example, in West German Publication No. 3113802 and U.S. Pat. Nos. 4,403,835 and 4,666,256. These objectives, in the same way as the objective described in West German Patent No. 1037719, provide a lens that is movable between two stationary lenses. This causes two air separations to change simultaneously: change in one air separation corrects for the spherical aberration introduced by the cover-glass, and change in the other air separation counteracts an impairment in the field correction for astigmatism, brought about by the movement.
It is also known from West German Patent No. 1037719 to move the entire objective simultaneously with a cover-glass thickness correction movement, for the purpose of resetting the focus.
In the power range of interest (a linear magnification between 32.times. and 60.times. and an aperture between 0.5 and 0.7), these measures can correct only for slight differences of about 1 mm in cover-glass thicknesses. The objective 40/0.55 indicated in West German Publication No. 3113802, for correcting cover-glass thickness differences of 2 mm, suffers from image errors, particularly astigmatism, which cannot be tolerated for a plano objective for a flat image field. In contrast, our way of setting a microscope objective to accommodate different cover-glass thicknesses can maintain a high quality correction of the image errors while accommodating a wide range of cover-glass thicknesses.